There is a growing awareness that wavelengths of sunlight are harmful to the eye. For example, light of the color blue, more more accurately stated, light with wavelengths between 400 nm and 515 nm are harmful over a long period of time. This is a rather surprising conclusion in view of the widespread presence of blue light, for example in the sky. However, tests have shown that blue light does gradually destroy the supply of visual purple which the retina of the human eye possesses, and can lead to senile macular degeneration, night blindness and snow blindness among other problems.
The adverse effects of light having wavelengths in the region between 400-515 nm are only recently beginning to be fully recognized and at least partially understood. For many years, persons in the outdoors have attempted to protect their eyes with "dark glasses", and such glasses, often called "sun glasses" have come in many colors, and with many explanations of their functions. Still, with only a few exceptions, such as special lenses for welders and skiers and filters for cameras and scientific purposes, their effort has been more to reduce the intensity of light transmitted, rather than to filter out any particular wavelength or group of wavelengths.
The special lenses which have existed have severely limited properties, because they are made of glass, and additives for the purpose of modifying the transmissivity and must be compatible with the high temperatures used to make the glass. The range of properties of such substances is very narrow, and does not approach the versatility of organic dyestuffs for this purpose. However, such dyestuffs are destroyed by the temperatures used in making the glass. Accordingly, while glass interference coated filters and lenses with special transmissive properties do exist, they are usually quite costly, and fail to provide the advantages of this invention as to low cost, and to versatility and breadth of function and beneficial effect.
A search of the prior art did not disclose any patents that read directly on the claims of the instant invention however, the following patents were considered related:
______________________________________ PATENT NUMBER INVENTOR ISSUED ______________________________________ 2,307,602 (German) Krumeich, et al 29 August 1974 3,745,032 Miller, et al 10 July 1973 3,571,649 Bush, Jr. 23 March 1971 3,460,960 Francel, et al 12 August 1969 ______________________________________
The Krumeich patent is primarily directed to correcting impaired color vision with lenses having a selective transmission function. The patent limits transmission in the blue range while allowing almost complete transmission in the red range. As can be seen in FIG. 1 of the patent, wavelengths less than about 490 nm are substantially cut-off having about 0.01% transmission. Light having wavelengths of about 550 nm or longer has at least about 10% transmission. Additionally, light having wavelengths of about 600 nm or longer has at least a 70% transmission.
The Miller patent discloses a glass composition that has improved ultraviolet and infrared transmittance characteristics. The invention is especially useful in manufacturing lenses for housing infrared and ultra-violet or radio-frequency detectors as are used in missile systems.
The Bush patent discloses selective color filtration devices for underwater viewing or photography. The patent discloses a predetermined color-selective light-filtering coating on a light source that is used for underwater viewing or photography. For example, to absorb light in the blue-green end of the visible spectrum and to pass the red, orange and yellow wavelengths, in accordance with a predetermined formula, the coating corrects to some extent the filtering effect caused by the water and by the particles suspended in the water. By following the teachings of the present invention, underwater objects may be photographed in more natural colors, and without the usual bluish-greenish tinge which normally accompanys objects that are photographed by present day equipment.
The Francel, et al patent discloses a method for coating a vitreous substrate with a fluid coating composition. The coating imparts to an otherwise clear and/or transparent glass substrate the light transmittance characteristics of an amber glass. Amber glass, which is widely used for a variety of applications such as reflectors, insulators and glass containers, inherently screens out appreciable percentages of lightwaves in the visible spectrum. For example, at 400 nm, the range of light transmission is between 1.53-7.0%; at 450 nm between 3.5-16.0%; and at 500 nm between 7.0-27.0%.